X-ray CT apparatus and X-ray imaging method

ABSTRACT

An X-ray CT apparatus includes a scanner mounted with a detection system having an X-ray source for generating X-rays applied radially to an object and a detector arranged so as to be opposite to the X-ray source and adapted to detect the image of the transmitted X-rays transmitted through the object, wherein the scanner is rotated around the object. A three-dimensional X-ray absorption coefficient distribution image of the object is reconstructed from the transmitted X-ray image and the rotation-axis projection position which is the position where the rotation center of the scanner is projected on the detection plane of a two-dimensional sensor constituting the detector is decided. On the basis of the contrast of the X-ray absorption coefficient distribution image reconstructed by using the decided rotation-axis projection position, the rotation-axis projection position is estimated, and an X-ray tomographic image or/and a three dimensional X-ray image of the object is generated.

TECHNICAL FIELD

The present invention relates to an X-ray computed tomography, and moreparticularly to the technique which is effectively applied to thepositioning of a rotation center of a scanner of a detection system in acone-beam X-ray computed tomography.

BACKGROUND ART

FIG. 6 is a view showing the general construction of a conventionalcone-beam X-ray CT. The conventional cone-beam X-ray CT was divided intoimaging unit 1 for carrying out the imaging and image processing unit 2for processing the detected image data. Control unit 3 carries out thewhole control for the imaging unit 1 and the image processing unit 2. Inthe imaging unit 1, an X-ray source 5 and a two-dimensional detector 6were arranged in such a way as to be opposite to each other through anobject. The X-ray source 5 and the two-dimensional detector 6 were botharranged in a scanner as a scanning mechanism which is rotated around anobject 7 with a central axis 9 for the rotation as the rotation center.

The scanner 4 was rotated every predetermined angle and thetwo-dimensional detector 6 carries out the measurement of the intensityof the transmitted X-rays, which were transmitted through the object 7,every predetermined angle, thereby carrying out the imaging of thetransmitted X-ray image of the object 7. The transmitted X-ray imagewhich has been imaged by the two-dimensional detector was converted intothe digital image data which was in turn outputted to the imageprocessing unit 2. But, in the following description, the angle ofrotation of the scanner 4 is referred to as a projection angle a.

In the image processing unit 2, first of all, the pre-processing such asthe gamma correction, the distortion correction, the logarithmictransformation and the non-uniformity correction of the two-dimensionaldetector 6 were carried out in pre-processing means 10. Next,reconstruction means 11, on the basis of all of the transmitted X-rayimages (all of the projected images) after completion of thepre-processing reconstructed the three-dimensional reconstructed imagewhich is the three-dimensional X-ray absorption coefficient distributionwithin the field of view of the object 7. As for this reconstructionarithmetic operation method, there is known the cone-beam reconstructionarithmetic operation method or the like by Feldkamp described in anarticle of L. A. Feldkamp et al.: PRACTICAL CONE-BEAM ALGORITHM, Journalof Optical Society of America, A.Vol. 1, No. 6, pp. 612 to 619 (1984)(article 1).

Finally, imaging means 12 subjected the three-dimensional reconstructedimage to the image processing such as the volume-rendering processing orthe maximum-intensity-projection processing of displaying the resultantimage in the form of the two-dimensional image on display means 13. Atthis time, on the basis of the parameters of a viewpoint, a region andthe like to be observed which has been inputted through instructionmeans (not shown) such as a keyboard, a mouse and a track ball, theimaging means 12 executed the image processing.

In the conventional cone-beam X-ray CT, the scanner 4 mounted with theimaging system including the X-ray source 5 and the two-dimensionaldetector 6 was rotated and the transmitted X-ray image obtained aroundthe object 7 was imaged, and the reconstruction means 11 obtained thethree-dimensional X-ray absorption coefficient distribution of theobject 7 placed on the stationary coordinate system fixed to theapparatus body. The stationary coordinate system was defined by theimaging system, i.e., the Z-axis as the rotation center 9 of the scanner4, and the rectangular Cartesian coordinates on the plane on which therotation orbit of an X-ray focus 14 of the X-ray source 5 lies(hereinafter, referred to as “a mid-plane” for short, when applicable),i.e., the X-axis and the Y-axis.

The position of an X-ray beam 8 imaged by the detection elements of thetwo-dimensional detector 6 was specified by an angle (projection angle)a between the straight line which passes through the orbit of the XYZcoordinate system for the X-ray focus 14 to reach the two-dimensionaldetector 6 and the X-axis, “the rotation-axis projection” which wasobtained by projecting the rotation center 9 on an imaginary plane(projection plane) 15 is placed on the incident plane of thetwo-dimensional detector 6, and “the mid-plane projection” which is thestraight line drew by the intersection between the mid-plane and theprojection plane. That is, the coordinate axes, when reconstruction thethree-dimensional X-ray absorption coefficient distribution of theobject 7, were the rotation axis projection and the mid-plane projectionon the projection plane.

Since for the actual imaging of the transmitted X-ray image, thecontinuous analog imaging is not carried out, but the discrete digitalimaging is carried out, when performing the reconstruction arithmeticoperation, the sampling pitch DP on the projection plane was alsorequired. In addition, a distance SOD extending from the X-ray focus 14to the rotation center 9, and a distance SID extending from the X-rayfocus 14 to the rotation-axis projection 17 were both required. In thefollowing description, the relative positional relationship among theX-ray focus 14, the two-dimensional detector 6 and the rotation center 9will be referred to as “the geometry of the imaging system”. Thegeometry of the imaging system is defined by the distance SOD extendingfrom the X-ray focus 14 to the rotation center 9, and the distance SIDextending from the X-ray focus 14 to the rotation-axis projection 17,the sampling pitch DP, the rotation-axis projection and the mid-planeprojection on the projection plane 15.

It is well known that of the parameters by which the geometry of theimaging system is determined, the higher accuracy is required for therotation-axis projection, the mid-plane projection and the samplingpitch than for the the distance SOD extending from the X-ray focus 14 tothe rotation center 9, and the distance SID extending from the X-rayfocus 14 to the 17 rotation-axis projection. For example, when theeffective aperture width of the two-dimensional detector 6 is 30 cm, andthe resolution thereof is 512×512 pixels, the accuracy of therotation-axis projection, the mid-plane projection and the samplingpitch DP required 0.1 pixel, i.e., about 0.05 mm. This reason is thateven if the fine error is present in the positions of the rotation-axisprojection and the mid-plane projection, and the sampling pitch DP, thereduction of the image quality is provided for the reconstructed image.

It is known that of the positions of the rotation-axis projection andthe mid-plane projection, and the sampling pitch DP, in particular, therotation-axis projection is important, and even if the fine error ispresent, generates the remarkable artifact in the reconstructed image.On the other hand, it was difficult to image directly the positions ofthe rotation-axis projection and the mid-plane projection, and thesampling pitch DP. This reason resulted from the fact that the positionsof the rotation-axis projection and the mid-plane projection, and thevalue of the sampling pitch DP depend on the characteristics of thetwo-dimensional detector 6 and the installation state of the apparatus.

As for a method of imaging the geometry of the imaging system with highaccuracy, there was “the X-ray Tomographic Imaging System” described inJP-A-9-173330 (article 2) by the same applicant. In the X-raytomographic imaging system described in the article 2, first of all, anobject (phantom) 19 including a support member 20 and a corpuscle-shapedhigh absorption member 21 shown in FIG. 7 is arranged in the vicinity ofa rotation center 9 (in the position which 3 cm to several centimeters away from the rotation center 9), and the transmitted X-ray image thereofis imaged from the all-round direction. But, in the followingdescription, the dedicated phantom 19, as shown in FIG. 7, which is usedin the correction of the geometry of the imaging system will referred toas “the geometry estimate phantom” or “the phantom” for short.

If after completion of the necessary pre-processing such as thedistortion correction and the non-uniformity correction, the transmittedX-ray images for all-round direction were added to each other, forexample, as shown in FIG. 8, each of the corpuscle-shaped highabsorption members 21 on the phantom 19 would draw an elliptical locus23 on an added image 34. Since the straight line passing through thecenters of the elliptical loci 23 becomes the rotation-axis projection17 depending on the imaging conditions for the geometry estimate phantom19, the position CP of the rotation-axis projection could be specified.On the other hand, the position MP of the mid-plane projection wasobtained from the change in the length of the diameter in the directionof the rotation center (the minor axis of the elliptical locus 23). Thatis, the lengths of the minor axes of a plurality of imaged ellipticalloci 23, and the positions of the rotation center directions thereofwere expressed in the form of the graph, and the position where thelength of the minor axis becomes zero was estimated, whereby theposition MP of the mid-plane projection was obtained.

For the sampling pitch DP, first of all, for example, a metallic platein which pin holes are bored at regular intervals, i.e., a hole chart orthe like is stuck as a thin object having a predetermined length on thelight receiving plane of the two-dimensional detector 6, i.e., theprojection plane 15 to image one sheet of transmitted X-ray image. Afterthe transmitted X-ray image had been subjected to the necessarypre-processing such as the distortion correction and the non-uniformitycorrection, with respect to how many pixels the image size or the holepart of the thin object corresponds to, the sampling pitch DP wasobtained by comparison with the actual size.

DISCLOSURE OF THE INVENTION

The present inventors, as a result of examining the prior art, found outthe following problems associated with the prior art. In theconventional X-ray CT, as described above, the estimation of thegeometry of the imaging system required much work of an operator, forexample, and much time. In particular, though the high accuracy wasrequired with respect to the position CP of the rotation-axisprojection, in the conventional geometry estimate method, themanipulation by an operator was required and hence there was a problemthat a burden was imposed on an operator.

In addition, the accuracy which was obtained by the conventional methodof estimating the geometry depended largely on a sense of an operator asa human being, and hence there was a problem that the sufficientaccuracy could not be obtained by an operator. In addition, since thework of specification or the like of each of the central positions ofthe elliptical loci 23 was required, the estimation of the geometry ofthe imaging system took a lot of time and hence there was a problem thatthe reduced diagnostic efficiency was shown.

As for the method of solving the above-mentioned problems, the geometryestimation described in the article 2 can also be automatically carriedout by executing the image recognition processing and the like. However,since in order to carry out the extremely accurate estimation, thecomplicated image processing needs to be performed, there is a problemthat the manufacture cost of the apparatus is increased.

It is an object of the present invention to provide the technique whichis capable of obtaining extremely accurately the position of therotation-axis projection which contributes largely to the promotion ofthe high quality image of a reconstructed image and also to provide thetechnique which is capable of estimating parameters used to define thegeometry of the imaging system without depending on a sense of anoperator, the technique which is capable of estimating automaticallyparameters used to define the geometry of the imaging system, and thetechnique which is capable of improving the diagnostic efficiency.

The objects and novel features of the present invention will be apparentby reference to the description of the present specification and theaccompanying drawings. Of the inventions disclosed herein, the typicalones are described as follows.

An X-ray CT according to the present invention includes: a scannermounted with a detection system having an X-ray source for applying theradial X-rays to a object and imaging means arranged so as to beopposite to the X-ray source and adapted to detect a transmitted X-rayimage of the X-rays transmitted through the object (phantom); rotationmeans for rotating the scanner around the object; reconstruction meansfor reconstructing a reconstructed image of the object from thetransmitted X-ray image; initial-value decision means for deciding aninitial value of the position of a rotation center (the position of arotation-axis projection) of the scanner which is projected on thetransmitted X-ray image, wherein the position of the rotation center ofthe scanner is estimated on the basis of the contrast of thethree-dimensional X-ray distribution image which is reconstructed bychanging the position of the rotation axis projection decided by theinitial value decision means; and an X-ray tomographic image or/and anX-ray three-dimensional image of the object is/are generated from thereconstructed image, which is reconstructed by utilizing the estimatedposition of the rotation center, to be displayed.

In addition, an X-ray imaging method according to the present inventionis an X-ray imaging method for obtaining an X-ray CT image, the methodincluding: the step of collecting a transmitted X-ray image of X-raystransmitted through a object by a scanner mounted with detection meanshaving an X-ray source for generating the X-rays applied radially to theobject (phantom) and imaging means arranged so as to be opposite to theX-ray source; the step of deciding previously the position of therotation-axis projection as the position which is obtained by projectinga rotation center of the scanner on a detection plane of atwo-dimensional detector constituting imaging means; the step ofreconstructing an X-ray absorption coefficient distribution image of theobject from the transmitted X-ray image on the basis of the position ofthe rotation-axis projection; the step of estimating the position of therotation-axis projection from the X-ray absorption coefficientdistribution image thus obtained; the step of reconstructing athree-dimensional X-ray absorption coefficient distribution image of theobject from the transmitted X-ray image; the step of generating an X-raytomographic image or/and the three-dimensional X-ray image of the objectfrom the three-dimensional X-ray absorption coefficient distributionimage thus obtained; and the step of displaying the X-ray tomographicimage or/and three-dimensional X-ray image thus obtained. For theestimation of the position of the rotation-axis projection, the positionof the rotation-axis projection where the contrast of the X-rayabsorption coefficient distribution image obtained from the transmittedX-ray image of the object shows a maximum or a local maximum isspecified and estimated as the position of the rotation-axis projectionon the transmitted X-ray image.

The property that if the reconstruction arithmetic operation is carriedout in the state in which the position of the projected rotation centeris deviated, since the artifact of arc shape is generated on theresultant reconstructed image, the contrast is reduced is utilized, andthe position of the rotation-axis projection where the contrast of thereconstructed image shows a maximum is decided as the proper position ofthe rotation-axis projection. As a result, the position of therotation-axis projection as the parameter used to define the geometry ofthe imaging system can be estimated using the value independent of asense of an operator and called the contrast of the reconstructed image.Therefore, the position of the rotation-axis projection whichcontributes greatly the promotion of the high image quality of thereconstructed image can be obtained with high accuracy.

In addition, the contrast of the reconstructed image is decided as theparameter used to define the geometry of the imaging system, whereby itbecomes possible to define the function of the contrast of thereconstructed image in which the projected rotation center is treated asthe variable. Therefore, it is possible to estimate automatically theposition of the projected rotation center as the parameter used todefine the geometry of the imaging system. As a result, a time requiredto estimate the geometry of the imaging system, i.e., a time required toadjust the X-ray CT can be reduced and hence the diagnostic efficiencycan be reduced.

The effects offered by the typical ones of the inventions disclosedherein are simply described as follows. (1) The position of therotation-axis projection which contributes greatly to the promotion ofthe high image quality of the three dimensional X-ray absorptioncoefficient distribution image can be obtained with high accuracy. (2)The parameters used to define the geometry of the imaging system can beestimated independently of a sense of an operator. (3) The parametersused to define the geometry of the imaging system can be automaticallyestimated. (4) The diagnostic efficiency can be enhanced.

The typical construction of the present invention is summarized asfollows with reference to FIG. 1. An X-ray CT includes: a scanner whichis mounted with an imaging system having an X-ray source for applyingthe radial X-rays to a object and imaging means arranged so as to beopposite to the X-ray source and adapted to detect a transmitted X-rayimage of the X-rays transmitted through the object; rotation means forrotating the scanner around the object; reconstruction means forreconstructing a reconstructed image of the object from the transmittedX-ray image; and decision means for deciding the position of therotation-axis projection as the position which is obtained by projectingthe rotation center of the scanner on a detection plane of atwo-dimensional X-ray detector constituting the imaging means, whereinthe position of the rotation-axis projection of the scanner is estimatedon the basis of the contrast of a reconstructed image which isreconstructed by using the rotation-axis projection decided by thedecision means, and an X-ray tomographic image or/and an X-raythree-dimensional image of the object is/are generated from thereconstructed image which is reconstructed in the position of theestimated rotation-axis projection to be displayed. According to thepresent invention, the position of the rotation-axis projection whichcontributes greatly to the promotion of the high image quality of thethree-dimensional absorption coefficient distribution image can beestimated with high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the schematic construction of a cone-beam X-rayCT as an X-ray CT of an embodiment 1 of the present invention;

FIG. 2 is a view useful in explaining the schematic construction of aphantom of the embodiment 1 of the present invention;

FIG. 3 is a view useful in explaining the geometry of an imaging systemas the relative positional relationship among an X-ray focus, atwo-dimensional detector and a rotation center;

FIG. 4 is a flow chart useful in explaining the procedure of estimatingthe geometry of the imaging system by the cone beam X-ray CT of theembodiment 1;

FIG. 5 is a view useful in explaining the schematic construction of aposition estimate phantom of the rotation axis projection used inestimation of the geometry of the imaging system in a cone-beam X-ray CTof an embodiment 2 of the present invention;

FIG. 6 is a view showing the schematic construction of a conventionalcone-beam X-ray CT;

FIG. 7 is a view useful in explaining the schematic construction of aconventional phantom; and

FIG. 8 is a view useful in explaining the procedure of estimating thegeometry of an imaging system by the conventional cone-beam X-ray CT.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will hereinafter be described withreference to the accompanying drawings. By the way, in all of thedrawings explaining embodiments of the present invention, constituentelements having the same functions are designated with the samereference numerals, and the repeated description thereof is omittedhere.

EMBODIMENT 1

FIG. 1 is a view showing the schematic construction of a cone-beam X-rayCT as an X-ray CT of an embodiment 1 of the present invention. Thecone-beam X-ray CT of an embodiment 1 includes imaging unit 1 forimaging a transmitted X-ray image obtained around an object 7, imageprocessing unit 2 for carrying out the image reconstruction from theimaged X-ray intensity image, and geometry estimate means (estimatemeans, and decision means for deciding the position of the rotation axisprojection) for estimating the geometry of an imaging system. Inaddition, the cone-beam X-ray CT includes also control unit 3 forcarrying out the whole control for the imaging unit 1, the imageprocessing unit 2 and the geometry estimate unit 24. But, the imagingunit 1 has the same construction as that of the conventional one andincludes a scanner (a rotating disc having a hollow central portion) 4which is mounted with a detection system having an X-ray source 5 forradiating the cone-beam-like X-rays, a two-dimensional X-ray detector(imaging means) 6 for imaging the transmitted X-ray image of the X-raystransmitted through an object 7, 31 to be imaged which is arranged inthe vicinity of a rotation center 9, and the X-ray source 5 which isarranged in such a way as to be opposite to the detector 6. The scanneris rotated around the rotation center 9 by rotation means (not shown).Next, the description will hereinbelow be given with respect to theprocessing executed in each of the constituent means and the flow of thedata transmitted between the constituent means on the basis of FIG. 1.But, the details of the individual processings will be described later.

The image processing unit 2 includes pre-processing means 10,reconstruction arithmetic operation means (reconstruction means) 11,imaging means 12 and display means 13. But, since the reconstructionmeans 11, the imaging means 12 and the display means 13 other than thepre-processing means 10 have the same configurations as those of theconventional cone-beam X-ray CT, the detailed description thereof isomitted here. In the normal reconstruction, similarly to theconventional pre-processing means, the pre-processing means 10 subjectsthe imaged image data to the necessary pre-processing such as the gammacorrection, the distortion correction, the logarithmic transformationand the non-uniformity correction. On the other hand, in the estimationof the geometry of the imaging system, the pre-processing means 10, onlyfor the previously specified area, subjects the imaged image data to thenecessary pre-processings such as the gamma correction, the distortioncorrection, the logarithmic transformation and the non uniformitycorrection. The previously specified area, for example, is set to theminimum area which is required to generate the projected imagecorresponding to the mid-plane projection (hereinafter, referred to as“the mid-plane area” for short, when applicable), whereby it is possibleto reduce a time required to estimate the Geometry of the imagingsystem. By the way, its details will be described later.

The pre-processing means 10 of the embodiment 1 can be constituted byfirst pre-processing means for subjecting the whole range of the imagedimage data to the pre-processing in the normal reconstruction, and bysecond pre-processing means for subjecting only the mid-plane area ofthe imaged image data to the pre-processings in the automatic estimationof the position CP of the rotation-axis projection.

Geometry estimate unit 24 includes initial-value decision means(initial-value memory means) 25 for deciding an initial value of theposition of the rotation-axis projection, mid-plane reconstruction means(partial reconstruction means) 26, evaluation-area decision means 27,evaluation-function calculation means 28 and optimizing means 29, and inparticular, carries out the estimation of the position CP of therotation axis projection of the geometry.

The initial-value decision means 25 for deciding an initial value of theposition of the rotation-axis projection is the means for obtaining aninitial value CP0 of the position CP of the rotation-axis projection.The initial-value decision means 25 of the embodiment 1 outputs as theinitial value CP0 the coordinate value at the center of the width in thedirection of the rotating tangential line of the projected image, i.e.,the coordinate value at the center of the aperture width of the detector6. The initial-value decision means 25 outputs the resultant initialvalue CP0 to the optimizing means 29.

The mid-plane reconstruction means 26 is the arithmetic operation meansfor carrying out the reconstruction arithmetic operation using theestimated value of the position CP of the rotation-axis projectionspecified by the optimizing means 29 to obtain a tomographic image onthe mid-plane 16 (hereinafter, referred to as “a mid-plane image” forshort, when applicable). In the embodiment 1, the mid-planereconstruction means 26 does not simply reconstruct the whole plane ofthe mid-plane, but reconstructs only the pre-decided area or theevaluation area decided by the evaluation-area decision means 27. Theprocedure of deciding the evaluation area by the evaluation areadecision means 27 will be described later.

While the embodiment 1 is constructed in such a way that the mid-planereconstruction means 26 obtains the mid-plane image, it is to beunderstood that the present invention is not intended to be limitedthereto, and for example, the reconstruction means 11 for carrying outthe normal reconstruction arithmetic operation may be employed as themid plane reconstruction means 26. In the case where the reconstructionmeans 11 is employed as the mid-plane reconstruction means 26, thereconstruction means 11, when carrying out the normal reconstruction,reconstructs the whole area of the reconstruction area specified by anoperator, and when executing the geometry estimate processing,reconstructs only the mid-plane cross section image, the pre-decidedarea or the evaluation area decided by the evaluation-area decisionmeans 27.

The evaluation-area decision means 27 is the means for when executingthe estimate processing for the position CP of the rotation-axisprojection, deciding the evaluation area becoming an object of theevaluation of the evaluation function. In the embodiment 1, in orderthat the burden imposed on an operator may be reduced and also theautomatic estimation for the position of the rotation center may bestably carried out, the evaluation-area decision means 27 carries outthe automatic decision of the evaluation area. But, the evaluation areaeither may be previously decided or may be specified on the basis of theinitially reconstructed image by an operator.

When utilizing the rotation-axis projection position estimate phantom 31for estimating the projected position of the rotation center, only theperiphery of the region in which the reconstructed image of the materialhaving the high absorption for the X-ray of the phantom 31 is presentcan be decided as the evaluation area. Then, the whole area of themid-plane cross section or the area within the range in which it isensured that the reconstructed image of an insert 32 having the highabsorption for the X-ray of the phantom 31 is surely present on themid-plane cross section is reconstructed on the basis of the initialvalue CP0 of the position of the rotation axis projection. Since in thisstage, the initial value CP0 does not correspond to the proper positionof the rotation axis projection, the artifact is contained in themid-plane reconstructed image and hence the contrast is generally low.However, even if it is assumed that the region showing a maximum valueof the reconstructed image is present in the vicinity of the regionhaving the reconstructed image of the insert 32 having the highabsorption for the X-ray of the phantom 31, there is no problempractically. In this connection, a maximum value of the CT value of thereconstructed image can be used as the contrast.

Therefore, in the embodiment 1, a predetermined area, containing thepoint at which the maximum value is taken, or having as the center thepoint at which the maximum value is taken, of the mid-plane image whichhas been reconstructed using the initial value CP0 is treated as theevaluation area, whereby the subsequent optimizing arithmetic operationcan be carried out.

The evaluation-function calculation means 28 is the means for evaluatingthe mid-plane tomographic image present in the evaluation area to obtainan evaluation function E(CP). The evaluation-function calculation means28 delivers the evaluation function E(CP) thus obtained to theoptimizing means 29. But, the evaluation function E(CP) will bedescribed later.

The optimizing means 29 is the means for controlling the flow of theprocessing of estimating the position CP of the rotation-axisprojection. The optimizing means carries out the decision and the updateof the estimated value for the position CP of the rotation-axisprojection, and delivers the estimated value for the position CP of therotation-axis projection to the mid-plane reconstruction means 26. Inaddition, the optimizing means 29 receives the evaluation function (CP)for the estimated value for the position CP of the rotation-axisprojection obtained by the evaluation-function calculation means 28. Theoptimizing means 29 records the change in the evaluation function E(CP)in relation to the change in the estimated value for the position CP ofthe rotation-axis projection, and on the basis of a trend of the changein the evaluation function E(CP), updates the estimated value for theposition CP of the rotation-axis projection to judge whether theprocessing of estimating the position CP of the rotation-axis projectionshould be carried out or the processing of estimating the position CP ofthe rotation-axis projection should be ended. Finally, the optimizingmeans 29 outputs as an optimized value CPf the value of the position CPof the rotation-axis projection where the evaluation function E(CP)shows a maximum value within a predetermined range of the error todeliver the optimized value CPf thus outputted to the image processingmeans 2. In this stage, the processing of estimating the position CP ofthe rotation-axis projection is ended. Subsequently, in the imageprocessing means 2, the reconstruction processing is executed using theoptimized value CPf of the position CP of the rotation axis projection.

FIG. 2 shows the phantom in the embodiment 1, in articular, thededicated phantom 31 which is suitable for the automatic estimation ofthe position CP of the rotation-axis projection (hereinafter, referredto as “the position estimate phantom of the rotation-axis projection” or“the phantom” for short, when applicable). In FIG. 2, reference numeral20 designates a support member, reference numeral 32 designates aninsert, and reference numeral 31 designates a body of the phantom. But,the phantom 31 is also claimed by the present invention. The phantom 31has the construction in which the insert 32 made of a material havinghigh absorption for the X-ray is embedded in the support member 20having low absorption for the X-ray. The support member 20 is formedinto a stick made of a material having a small X-ray absorptioncoefficient such as a plastic material or polymeric resin includingacrylate resin, vinyl chloride, polycarbonate or the like, or material,typified by wood, through which the X-ray is transmitted and which hasthe high strength against the mechanical destruction. The insert 32 isthe columnar body with a predetermined length which is made of amaterial having a large X-ray absorption coefficient such as tungsten,platinum or an iron-nickel-chromium series alloy, and is formed into awire or a stick.

The phantom 31 is preferably constructed in such a way that the insert32 which is made of an iron-nickel-chromium series alloy and which isformed into a stick having a diameter of about 0.5 mm to about 1 mm isembedded along the axial direction of the stick-shaped support member 20having a diameter of about 1 cm. In addition, since the insert 32, wheninstalling the phantom 31 in the vicinity of the rotation center 9,needs to be set so as to cross the mid-plane, the insert 32 needs tohave the length which is roughly equal to or longer than can be set withthe naked eye. But, it is to be understood that the materials for thesupport member 20 and the insert 32 may be any of materials other thanthe above-mentioned materials as long as the desired contrast isobtained. Moreover, the support member 21 is provided in order toprevent the deformation or the like of the insert 32, or in order, tomake easy the installation of the phantom in a predetermined position.Therefore, it is to be understood that even when only the insert 32 isprovided for the phantom 31, it may also be available.

Since in the phantom 31 of the embodiment 1, the insert 32 having thehigh absorption for the X-ray is formed into a wire or a stick, whenreconstructed in the form of the mid plane cross section, itsreconstructed image shows a point and hence the evaluation of thecontrast becomes easy. In addition, since when set in a predeterminedposition, the phantom 31 extends with a predetermined length along thedirection which is roughly parallel to the rotation center 9, even ifthe phantom 31 is slightly deviated in the direction of the rotationcenter in the installation thereof, the deviation does not exert aninfluence on the reconstructed image when carrying out the imaging. Thatis, since the phantom is made of the above-mentioned material and alsohas the above-mentioned shape, the imaging and the evaluation of thecontrast become easy to be carried out.

In addition, if the phantom 31 of the embodiment 1 is utilized, then theevaluation area can be further restricted. That is, the whole area ofthe mid-plane cross section for which the reconstruction arithmeticoperation can be carried out does not need to be reconstructed, andhence only the periphery of the region in which the constructed image ofthe insert 32 having the high absorption for the X-ray of the phantom ispresent on the mid-plane cross section has only to be reconstructed.Since the area to be reconstructed is restricted to a minimum, it is notinfluenced by the region, which is inconvenient for the evaluation ofthe contrast, of a part constituting the phantom 31. By the partconstituting the phantom is, for example, meant the jigs, the screws orthe like for supporting the phantom 31. In addition, if the evaluationarea is restricted, then the reconstruction arithmetic operation volumemay also be a necessary minimum. As a result, the evaluation of thecontrast can be carried out under the more stable condition. Asdescribed above, the phantom 31 is utilized in the estimation of thegeometry of the imaging system, whereby the evaluation of the contrastbecomes easy on the reconstructed image.

Next, a view useful in explaining the relative positional relationshipamong the X-ray focus 14, the two-dimensional detector 6, and therotation center 9, i.e., and the geometry of the imaging system, and thegeometry of the imaging system will hereinbelow be described withreference to FIG. 3. But, in the following description, the constituentelements of the imaging system are abstracted, and the imaginary planeis placed in the position of the two-dimensional detector 6, and thisplane is referred to as the projection plane. The plane on which therotation orbit of the X-ray focus 14 lies is the mid-plane 16, and thestraight line which is formed by projecting the rotation center 9 on theprojection plane 15 is the rotation axis projection 17. The straightline which is formed by the projecting the mid-plane on the projectionplane 15, i.e., the line of intersection between the mid-plane 16 andthe projection plane 15 is the mid-plane projection 18.

The geometry of the imaging system is defined by a distance SID betweenthe X-ray focus 14 and the projection plane 15, a distance SOD betweenthe X-ray focus 14 and the rotation center 9, the position CP of therotation-axis projection on the projection plane 15, and the position MPof the mid-plane projection. In addition, since in the actual imaging,the discrete imaging is carried out, when carrying out thereconstruction arithmetic operation, a sampling pitch DP on theprojection plane 15 is also required. As described above, with respectto the distance SID between the X-ray focus 14 and the projection plane15, and the distance SOD between the X-ray focus 14 and the rotationcenter 9 of the parameters used to decide the geometry, even if theerror is slightly contained therein, the image quality of thereconstructed image which is finally obtained is not remarkably reduced.Therefore such parameters can be directly measured after completion ofthe assembly of the apparatus. Or, the apparatus is assembled within therange of the mechanical error specified in the designing stage, wherebythe distances SID and SOD can be determined.

On the other hand, with respect to the position CP of the rotation-axisprojection, the position MP of the mid-plane projection and the samplingpitch DP, the higher accuracy is required as compared with the distancesSID and SOD. For example, if the effective aperture width of thedetector 6 is 30 cm and the resolution is 512×512 pixels, for the CP, MPand DP, the accuracy of the 0.1 pixel, i.e., about 0.05 mm is required.This requirement results from the fact that even if the fine error ispresent in the position CP of the rotation-axis projection, the positionMP of the mid-plane projection and the sampling pitch DP, the reductionof the image quality is provided in the reconstructed image. Inparticular, the position CP of the rotation-axis projection of thegeometry of the imaging system, is important, and thus even if the fineerror is present therein, the remarkable artifact is generated in thereconstructed image. However, it is difficult to measure directly thesevalues. This results from the fact that the values of the position CP ofthe rotation-axis projection, the position MP of the mid-planeprojection and the sampling pitch DP depend on the characteristics ofthe detector 6 and the installation state of the apparatus.

Next, the flow useful in explaining the procedure of estimating thegeometry of the imaging system by the cone-beam X-ray CT of theembodiment 1 is shown in FIG. 4. The operation of estimatingautomatically the geometry of the imaging system by the cone-beam X-rayCT of the embodiment 1 will hereinbelow be described on the basis of theflow shown in FIG. 4. But, since both of the mid-plane 16 and themid-plane projection 18 are the same as those in the prior art, thedetailed description thereof is omitted here.

(Step 401)

First of all, the phantom 31 is installed in a predetermined position tocarry out the cone-beam imaging to collect the transmitted X-ray image(imaged image data) obtained by the rotation made around the object.

(Step 402)

Next, the pre-processing means 10 subjects the imaged image dataobtained by the rotation made around the object to the necessarypre-processing such as the gamma correction, the distortion correction,the logarithmic transformation and the non-uniformity correction togenerate the projected image. At this time, only the data within theregion required to generate the projected image on the position of themid-plane projection has only to be subjected to the pre-processings.

(Step 403)

The initial-value decision means 25 decides the initial value CP0 of theposition CP of the rotation-axis projection. As the initial value CP0,for example, the coordinate at the central position of the aperturewidth of the detector 6 is employed.

(Step 404)

The mid-plane reconstruction means 26 reconstructs the mid-planetomographic image or a predetermined area on the mid-plane by utilizingthe initial value CP0.

(Step 405)

The evaluation-area decision means 27 detects the pixel position showinga maximum on the mid-plane tomographic image obtained in Step 404 to setas the evaluation area the area containing that position or the areawithin a predetermined range with that position as the center.

(Step 406)

The reconstruction of the evaluation area and the calculation of theevaluation function are repeatedly carried out to perform theoptimization within the range of a predetermined error e to estimate theposition CP of the rotation-axis projection. But, the meaning of theoptimization is to obtain the position CP of the rotation-axisprojection where the evaluation function E(CP) shows a maximum, i.e., toobtain the optimized value CPf. More specifically, the optimization iscarried out as follows.

(Step 407)

The evaluation area on the mid-plane is reconstructed with the estimatedvalue CPk as the position of the rotation-axis projection to obtain themid-plane tomographic image.

(Step 408)

For the mid-plane tomographic image obtained in Step 407, an evaluationfunction Ek=E(CPk) is obtained.

(Step 409)

The position CP of the rotation-axis projection where the evaluationfunction E(CP) shows a maximum is estimated from the series of E0, E1,E2, . . . , Ek to be decided as CP{k+1}.

(Step 410)

When (Expression 1) is fulfilled with e as a pre-set error, k isincremented to (k+1) using the estimated value CP {k+1} to executerepeatedly Step 407 to Step 410.

|CPk−CP{k+1}|>e  (Expression 1)

On the other hand, when (Expression 2) is fulfilled, the processingproceeds to next Step 411.

|CPk−CP{k+1}|≦e  (Expression 2)

(Step 411)

CP{k+1} is outputted as the optimized value CPf.

The foregoing is the flow of the automatic estimation of the position CPof the rotation-axis projection utilizing the cone-beam X-ray CT of theembodiment 1 as the typical embodiment of the present invention.

In the cone-beam X-ray CT of the embodiment 1, the reconstructionarithmetic operation is carried out in the reconstruction means 11 usingthe optimized value CPf of the position CP of the rotation-axisprojection which has been estimated on the basis of the estimateprocessing shown in Step 401 to Step 411. The procedure of estimatingthe rotation-axis projection using the transmitted X-ray image of thephantom 31 in the X-ray CT of the embodiment 1 is as follows.

The estimate procedure includes: the step of collecting the detectedimage data as the transmitted X-ray image (Step 401); the step ofsubjecting the imaged image data to the pre-processing correction (Step402); the step of determining the initial value CP0 of the position CPof the rotation-axis projection by the initial-value decision means 25(Step 403); the step of on the basis of the rotation center positionpre-set by the initial-value decision means 25, reconstructing themid-plane tomographic image of a object from the imaged image data bythe mid-plane reconstruction means 26 (Step 404); the step of specifyingthe position, where the contrast of the mid-plane tomographic image isincreased, as the rotation center position of the detection systemprojected on the transmitted X-ray image (i.e., the rotation centerposition of the scanner); the step of deciding as the evaluation areathe area containing the maximum value pixel position or the area havingthe maximum value pixel position as the center by the evaluation-areadecision means 27 (Step 405); the step of carrying out repeatedly thereconstruction of the evaluation area and the calculation of theevaluation function to carry out the optimization within the range ofthe predetermined error e to estimate the position CP of therotation-axis projection (Step 406); the step of generating the X-raytomographic image or/and the X-ray three-dimensional image from thereconstructed image reconstructed using the estimated position CP of therotation-axis projection; and the step of displaying the X-raytomographic image or/and the X-ray three-dimensional image whichhas/have been generated.

In addition, while in the imaging unit 1 of the cone-beam X-ray CT ofthe embodiment 1, there is adopted the construction in which the object7 is fixed to a bedstead (not shown), and the scanner 4 mounted with theX-ray source 5 and the detector 6 are rotated around the object 7, theimaging unit 1 may have the construction other than the above-mentionedone. For example, there is conceivable the cone-beam X-ray CT includingthe imaging unit 1 having the construction in which both of the X-raysource 5 and the two-dimensional detector 6 are fixed by a supportingarm (not shown) or the like and in the imaging, the object 7 is rotated.In this case, since it becomes possible to change readily the geometryof the imaging system, the automatic estimation of the position of therotation-axis projection according to the present invention is moreeffective.

(Estimate Arithmetic Operation of Position CP of Rotation-AxisProjection)

Next, the description will hereinbelow be given with respect to theestimate arithmetic operation of the position CP of the rotation-axisprojection which is employed in the cone-beam X-ray CT of theembodiment 1. First of all, the basic principles of estimatingautomatically the position CP of the rotation center which is realizedby the present invention will now be described. In general, it is knownthat if the reconstruction arithmetic operation is carried out with theposition CP of the rotation-axis projection deviated, then artifact ofarc shape is generated in the resultant reconstructed image, and alsothe contrast is reduced. That is, when the reconstruction arithmeticoperation is carried out using the proper position CPp of therotation-axis projection, the contrast of the reconstructed imagebecomes a maximum.

Therefore, in the present invention, the position CP of therotation-axis projection where the contrast of the reconstructed imagebecomes a maximum is obtained, whereby the proper position CPp of therotation-axis projection is calculated. To put is concretely, theposition CP of the rotation center is made a variable, and thereconstructed tomographic image within a predetermined concern area isreconstructed using the position CP of the rotation center. If theevaluation function E(CP) used to evaluate the contrast of thereconstructed tomographic image is considered, when the position CP ismade a variable, CP where the evaluation function E(CP) is made amaximum becomes the proper position CPp of the rotation center.

The method of obtaining a variable with which a predetermined functionis made a maximum (or a minimum in some cases) is referred to as “theoptimization” in the field of the numerical calculation, and thus iswell known. Therefore, the automatic estimation of the position CP ofthe rotation-axis projection results in the problem of the optimizationof the evaluation function E(CP). But, in general, it is known that theoptimization which does not entail the error at all can not be carriedout by the finite arithmetic operation. On the other hand, since in theX-ray detection, the error is contained in the imaged image data itselfbecoming the source of the optimization, in the present invention, theoptimized value CPf=CPp is obtained within the range of thepredetermined error e which is previously decided.

After completion of the formulation, the next problem is theoptimization method and the method of calculating the evaluationfunction E(CP). As for the optimization method, the various kinds ofmethods have already been proposed in the field of the numericalcalculation. Therefore, in the following description, the method ofapplying the well known optimization method and the evaluation functionto the present invention will hereinbelow be described.

The simplest optimization method is the method wherein thereconstruction arithmetic operation is successively carried out whilechanging the position CP of the rotation axis projection from theinitial value CP0 a small change amount dCP by a small change amountdCP; the evaluation function E(CP) for the resultant reconstructed imageis calculated; and the position CP of the rotation-axis projection wherethe value of the evaluation function E(CP) becomes a maximum is decidedas the optimized value CPf.

That is, when an arbitrary position of the rotation-axis projection isCpi, and its evaluation function is Ei, the arbitrary position Cpi ofthe rotation-axis projection, when i=0, 1, . . . , n is expressed by(Expression 3).

Cpi=CP0+I×dCP  (Expression 3)

Therefore, the evaluation function Ei=E(Cpi) for each of i=0, 1, . . . ,n is obtained, and the position Cpi where the evaluation function Eibecomes a maximum is obtained, whereby the optimized value CPf of theposition of the rotation-axis projection is obtained. In this case, theposition CP of the rotation-axis projection can not be estimated withhigher accuracy than the change amount dCP. Then, the optimized valueCPf of the position of the rotation-axis projection for the changeamount dCP is temporarily estimated on the basis of the above-mentionedprocedure. Next, the change amount of the position CP of therotation-axis projection is newly obtained by (Expression 4), and theinitial value thereof is obtained by (Expression 5).

DCP′=dCP/n  (Expression 4)

CP 0 ′=Cpi−dCP′×n/2  (Expression 5)

Thereafter, similarly to the above-mentioned process, the value of theposition CP of the rotation-axis projection is changed from the initialvalue CP0′ the change amount dCP′ by the change amount dCP′ ((Expression6)), the position Cpi′ of the rotation-axis projection where theevaluation function E(Cpi′) shows a maximum is obtained, and theresultant position Cpi′ of the rotation-axis projection is decided as anew optimized value CPf′.

Cpi′=CP 0 ′+i×dCP′  (Expression 6)

The same processing is repeatedly executed until the minute changeamount of the position CP of the rotation-axis projection falls withinthe desired error, whereby the optimized value CPf of the position ofthe rotation-axis projection which has the desired accuracy can beobtained. But, in general, since the reconstruction arithmetic operationis expensive in calculation cost, i.e., it takes a lot of time to carryout the reconstruction arithmetic operation, it is better that thenumber of times of reconstruction arithmetic operation is small.Therefore, the optimized number of times of reconstruction arithmeticoperation needs to be found out on the basis of the experimentation andthe like.

(Evaluation Function)

Next, the method of calculating the evaluation function E(CP) willhereinbelow be described. As described above, when the position CP ofthe rotation-axis projection is deviated, the influence appears in whichthe artifact of arc shape is generated in the reconstructed image toreduce the contrast. By utilizing this property, there can be decidedthe evaluation function E(CP) which takes a maximum value (or a minimumvalue) when the value of the position CP of the rotation-axis projectionbecomes the proper value. Practically, not only the evaluation functionE(CP) is simply decided, but also the imaging condition, the imagingobject and the concern area to be reconstructed are suitably adjusted insuch a way that the calculation of the evaluation function E(CP) becomeseasy to be carried out, whereby it is possible to reduce a time requiredto estimate the geometry of the imaging system. But, in the followingdescription, the concern area which is to be reconstructed and becomesan object of the evaluation is particularly referred to as “theevaluation area”. In addition, in the present invention, the maximumvalue in the evaluation area which is obtained by the reconstruction isdecided as the evaluation function E(CP) for the specific position CP ofthe rotation-axis projection.

In general, in the reconstructed image having high contrast, the maximumvalue in the reconstructed image becomes larger. Therefore, if themaximum value of the evaluation area is decided as the evaluationfunction E(CP), then it will show the value for which the fluctuationsof the contrast are reflected. But, in the scope of claims of thepresent invention, it is to be understood that any of the functionswhich has a maximum value or a minimum value in the standard deviation,the mean value or the minimum value of the evaluation area, or the valuewhich is calculated on the basis thereof, or the optimized value CPf maybe decided as the evaluation function E(CP). In addition, it is also tobe understood that the detection or the like of the artifact based onthe image recognition is carried out, and on the basis of the detectionresult, the evaluation function E(CP) may be obtained.

In order to calculate the evaluation function E(CP), i.e., the contrastof the reconstructed image, it is not necessary to reconstruct the wholearea for which the reconstructed arithmetic operation can be made, butonly the specific area fulfilling the conditions which will be describedlater has only to be reconstructed. Therefore, in the present invention,the whole area for which the reconstruction arithmetic operation can bemade is not decided as the evaluation area, but only the mid-plane crosssection is decided as the evaluation area. In order to calculate theevaluation function E(CP), i.e., the contrast of the reconstructedimage, the whole area for which the reconstruction arithmetic operationcan be made does not need to be reconstructed, but only the specificarea has only to be reconstructed. At this time, if only thereconstruction arithmetic operation for only the mid-plane cross sectionis carried out, then the operation volume required for thereconstruction arithmetic operation is kept to a minimum in terms of theproperty of the cone-beam reconstruction arithmetic operation byFeldkamp. In addition, since for the necessary projected image, only theprojected image on the mid-plane projection has only to be require, thenecessary pre-processing such as the gamma correction, the distortioncorrection, the logarithmic transformation and the non-uniformitycorrection is kept to a minimum. Therefore, it becomes possible toreduce largely a time required for the estimation of the geometry of theimaging means, and as a result it becomes possible to enhance thediagnostic efficiency.

(Other Estimate Operation for Position CP of Rotation-Axis Projection)

As for the optimization method of obtaining more efficiently theoptimized position CP of the rotation-axis projection, the arithmeticoperation method referred to as the well known “Brent method” can beused. “The Brent method” is realized by combining the arithmeticoperation method referred as to “the golden section means” and thearithmetic operation method referred to as “the parabolic interpolationmeans” with each other and thus is the optimization method wherein theoptimized value, i.e., the position CP where the evaluation functionE(CP) shows a maximum value can be obtained with the less operationvolume. The golden section means is the method wherein the range inwhich the presence of the optimized value is estimated is successively,surely narrowed. On the other hand, the parabolic interpolation means isthe method wherein the work of applying a parabola to the given threepoints to obtain the vertex of the parabola to further apply a parabolato the resultant vertex and the two points of the given three points isrepeatedly carried out to obtain the optimized value at high speed.While the optimized value can be found out more speedily in theparabolic interpolation means than in the golden section means, theparabolic interpolation means may not be applied depending on thecondition in some cases.

The Brent method is the practical method in which the golden sectionmeans and the parabolic interpolation means are combined with eachother. With respect to the details of the Brent method, for example,refer to an article of Willam H. Press et al.: “NUMERICAL RECIPES IN C”,Cambridge University Press, Second Edition, pp. 402 to 405 (1992)(“NUMERICAL RECIPE IN C” Japanes version by Gijyutsu hyoron sha, pp. 289to 292 (1993)) or the like. Of course, it is to be understood that anyof the optimization methods other than the above-mentioned methods maybe utilized in order to obtain the optimized value CPf.

(Procedure of Reconstructing Three-dimensional X-ray AbsorptionCoefficient Distribution Image)

Next, the description will hereinbelow be given with respect to theprocedure of obtaining the X-ray absorption coefficient distributionimage, i.e., the reconstructed image by utilizing the position CP of therotation-axis projection which is estimated as described above. First ofall, it is assumed that the necessary pre-processing in this stage,i.e., the gamma correction, the distortion correction, the logarithmictransformation and the non-uniformity correction for the imaged imagedata are carried out to obtain all of the projected images. Then, thereconstruction arithmetic operation is carried out on the basis of allof the projected images to obtain the reconstructed image. As for thereconstruction arithmetic operation processing therefore, there is knownthe cone-beam reconstruction arithmetic operation method by Feldkampdescribed in the article 1.

The description will hereinbelow be given on the basis of the geometryof the imaging system shown in FIG. 3. While in the reconstructionarithmetic operation described in the article 1, the arithmeticoperation is carried out on the basis of a projection angle a, thecoordinate (u, v) on the projection plane, and the coordinate (x, y, z)in the reconstruction space, the correspondence relationship betweenthese coordinates and the projected image obtained from the imagedimageing data which is actually imaged must be clear because theabove-mentioned projected image actually obtained is the discretelysampled data.

The ideal projected image is expressed in the form of P(a, u, v) byemploying the projection angle a and the position u, v on the projectionplane. On the other hand, the actually obtained projected image isdecided as Pr{I, j, k}. Since the latter projected image is discretelysampled, the indexes i, j and k take the integral numbers of i=0, 1, . .. , N-1, j=0, 1, . . . , M-1 and k=0, 1, . . . , L-1, respectively.Also, N means the number of projections, and M and N mean the resolutionin the direction of u, and the resolution in the direction v,respectively. In actual, this projected image corresponds to the datastored in the specific location on a memory which is specified by theindexes i, j and k. At this time, the following relationship of(Expression 7) is established between P(a, u, v) and Pr{i, j, k}. But, afunction Int(x) in (Expression 7) is a function of omitting the figuresof x below the decimal point.

P(a, u, v)=Pr{nt(a/dA), Int(u+CP)/dU, Int((v+MP)/dV)}  (Expression 7)

From this (Expression 7), the coordinates used in the reconstructionarithmetic operation and the discretely sampled projected image obtainedfrom the actual imaged image data is established. In the reconstructionarithmetic operation, the discretely sampled projected image may beutilized on the basis of (Expression 7). Of cause, the reconstructionarithmetic operation method from the projected image is not intended tobe limited to only the reconstruction arithmetic operation method byFeldkamp described in the article 1. However, no matter whatreconstruction arithmetic operation method we may employ, the geometryof the imaging means is the basis of the arithmetic operation, and henceit remains unchanged that the correspondence relationship between theprojected image which was obtained from the actual imaged image data andthe coordinates in the reconstruction arithmetic operation isestablished.

As described above, in the cone-beam X-ray CT of the embodiment 1, thereis utilized the fact that since if the reconstruction arithmeticoperation is carried out in the state in which the position of therotation-axis projection 17 projected on the projection plane 15 isdeviated, then the artifact of arc shape is generated in the resultantreconstructed image, the contrast is reduced. That is, first of all, themid-plane reconstruction means 26 carries out the reconstruction of thereconstructed image with the value decided by the initial-value decisionmeans 25 as the initial value of the position of the rotation-axisprojection, and then the evaluation-function calculation means 28calculates the evaluation function E(CP0) corresponding to the contrastof the reconstructed image. Next, the optimizing means 29 updates theinitial value of the position of the rotation-axis projection, themid-lane reconstructed image based on the updated value of the positionof the rotation-axis projection, the evaluation-function calculationmeans 28 calculates the evaluation function E(CP1) corresponding to thecontrast of the reconstructed image in the updated value of the positionof the rotation-axis projection, and the optimizing means compares theevaluation functions E(CP0) and E(CP1) with each other. Then, theabove-mentioned operation is repeatedly carried out to calculate theposition of the rotation-axis projection 17 where the contrast becomes amaximum, whereby it is possible to obtain the proper position CPp of therotation-axis projection. As a result, the rotation-axis projection 17as the parameter used to define the geometry of the imaging system canbe automatically estimated independently of a sense of an operator byusing the value which is independent of a sense of an operator as thecontrast of the reconstructed image. Therefore, a time required for theestimation of the geometry of the imaging system, i.e., a time requiredto adjust the X-ray CT can be reduced and hence the diagnosticefficiency can be enhanced. In addition, since the parameter used todefine the geometry of the imaging system can be automatically estimatedindependently of a sense of an operator, the position of therotation-axis projection which contributes greatly to the promotion ofthe high image quality of the reconstructed image can be obtained withhigh accuracy.

EMBODIMENT 2

FIG. 5 is a view useful in explaining the schematic construction of thephantom for use in the estimation of the geometry of the imaging systemin a cone-beam X-ray CT of an embodiment 2. In the embodiment 1employing the phantom 31 shown in FIG. 2, the detection for theestimation of the position of the mid-plane projection, and thedetection for the estimation of the position of the rotation-axisprojection must be carried out separately from each other. On the otherhand, the embodiment 2 relates to the phantom for concluding thedetection for the estimation of the position of the mid-plane projectionand the detection for the estimation of the position of therotation-axis projection by one time.

In FIG. 5, reference numeral 20 designates a support member, referencenumeral 21 designates a corpuscle-shaped high absorption member,reference numeral 32 designates an insert, and reference numeral 31designates a body of a phantom. As shown in FIG. 5, the phantom 31 ofthe embodiment 2 is such that the phantom, shown in FIG. 2, in which theinsert 32 made of the material having the high X-ray absorption isembedded in the support member 20 having the low X-ray absorption(hereinafter, referred to as “the first phantom” for short, whenapplicable), and the phantom, shown in FIG. 7, in which thecorpuscle-shaped high absorption members 21 each having the diameter ofabout 1 mm to about 2 mm each of which is made of the material havingthe high X-ray absorption are embedded at the intervals of about 2 cm inthe support member 20 having the low X-ray absorption along the axialdirection of the support member 20 (hereinafter, referred to as “thesecond phantom” for short, when applicable) are fixed with a secondsupport member 22. But, the phantom 31 shown in FIG. 5 is also claimedby the present invention.

The support member 20 is formed into a stick made of a material having asmall X-ray absorption coefficient such as a plastic material orpolymeric resin including acrylate resin, vinyl chloride, polycarbonateor the like, or a material, typified by wood, through which the X-ray istransmitted and which has the high strength against the mechanicaldestruction. The corpuscle-shaped high absorption member 21 is made of amaterial having the high X-ray absorption coefficient such as tungsten,platinum or an iron-nickel-chromium series alloy. As for the number ofcorpuscle-shaped high absorption members 21, for example, two or more isrequired. In addition, while the positions of the corpuscle-shaped highabsorption members 21 are arbitrary as long as they are arranged alongthe axis of the support member 20, the corpuscle-shaped high absorptionmembers 21 are arranged in such a way that when the phantom 31 isinstalled in such a way that at least the insert 32 crosses themid-plane 16, at least two corpuscle-shaped high absorption members 21are arranged so as to sandwich therebetween the mid-plane 16, wherebythe geometry of the imaging system can be carried out with one detectionof the transmitted X-ray image made around the object. The insert 32 isthe columnar body with a predetermined length which is made of amaterial having large X-ray absorption coefficient such as tungsten,platinum or an iron-nickel-chromium series alloy, and is formed into awire or a stick. Similarly to the support member 20, the second supportmember 22 is formed into a stick made of a material having a small X-rayabsorption coefficient such as a plastic material or a polymeric resinincluding acrylate resin, vinyl chloride, polycarbonate or the like, ora material, typified by wood, through which the X-ray is transmitted andwhich has the high strength against the mechanical destruction. In thecone-beam X-ray CT in the embodiment 2, by employing the second phantomas described above, the estimation of the position MP of the mid-planeprojection and the estimation of the position CP of the rotation-axisprojection can be continuously carried out on the basis of the imageddata obtained by one detection. The estimation of the position MP of themid-plane projection is as shown in the prior art method.

As described above, in the cone-beam X-ray CT apparatus of theembodiment 2, there is offered the effect that the transmitted X-rayimage which is obtained around the object by employing the phantom 31shown in FIG. 5 is detected, whereby next to the estimation of theposition MP of the mid-plane projection, the estimation of the positionCP of the rotation axis projection can be continuously carried out fromthe imaged data obtained one and the same detection of the phantom.

The X-ray CT of each of the embodiments 1 and 2 may include themid-plane projection memory means 30 as the memory means (temporarystorage means) for storing a part of the projected image which has beensubjected to the processing by the pre-processing means 10 for a periodof time when executing the geometry estimate processing. Since theprojected image required for the geometry estimation may be only thepart corresponding to the mid-plane projection 18, for example, thememory means employing the well known semiconductor memory is prepared,as the memory means from which the data can be read out at high speed,in the form of the mid-plane projection memory means 30, separately fromthe memory means for storing therein the whole projected image, wherebythe mid-plane reconstruction can be carried out speedily. As a result, atime required for the estimation of the geometry of the imaging systemcan be further reduced.

As the two-dimensional detector 6, there is used the X-ray imageintensifier-TV camera system or the two-dimensional X-ray detector inwhich the photodiodes, the TFT switches and the like are arranged in atwo-dimensional manner. In addition, in particular, in the case wherethe present invention is applied to the medical care X-ray CT in whichthe human body is treated as the detection object as the object, theeffect inherent therein can be obtained. In this case, since the highimage quality of the X-ray tomographic image or/and thethree-dimensional X-ray image which is/are obtained can be realized, thediscovery of the relatively small tumors or the like such as the earlycancer becomes easy, and hence the diagnostic accuracy as well as thediagnostic efficiency can be enhanced. But, it is to be understood thatas typified by the goods, in the case where any of the things other thanthe human body is treated as the object, the present invention can alsobe applied thereto.

From the foregoing, it is to be understood that the present invention isnot intended to be limited to the embodiments thereof and hence may bevariously changed without departing from the object matter of theinvention.

The meanings of the reference numerals used in the description of thedrawings are as follows: 1: imaging unit, 2: image processing unit, 3:control unit, 4: scanner (rotating disc), 5: X-ray source, 6: twodimensional detector, 7: object, 8: X-ray beam, 9: rotation center, 10:pre-processing means, 11: reconstruction means, 12: imaging means, 13:display means, 14: X-ray focus, 15: projection plane, 16: mid-plane, 17:rotation-axis projection, 18: mid-plane projection, 19: geometryphantom, 20: support member, 21: corpuscle-shaped high absorptionmember, 22: second support member, 23: elliptical locus, 24: geometryestimate means, 25: initial-value decision means, 26: mid-planereconstruction means, 27: evaluation-area decision means, 28:evaluation-function calculation means, 29: optimizing means, 30:mid-plane projection memory means, 31: phantom, 32: insert, 34: addedimage.

What is claimed is:
 1. An X-ray CT apparatus comprising: a scanner mounted with a detection system having an X-ray source for generating X-rays applied radially to an object and detection means arranged so as to be opposite to said X-ray source and adapted to detect the image of the transmitted X-rays transmitted through said object; rotation means for rotating said scanner around said object; reconstruction means for reconstructing a three-dimensional X-ray absorption coefficient distribution image of said object from said transmitted X-ray image; decision means for deciding the rotation-axis projection position which is the position where the rotation center of said scanner is projected on the detection plane of a two-dimensional sensor constituting said X-ray absorption coefficient distribution image reconstructed by using said rotation-axis projection position decided by said decision means, said rotation-axis projection position is estimated; an X-ray tomographic image or/and a three-dimensional X-ray image of said object is generated from said three-dimensional X-ray absorption coefficient distribution image reconstructed by said reconstruction means in said estimated rotation-axis projection position; and the X-ray tomographic image or/and the three-dimensional X-ray image of said object is/are displayed.
 2. An X-ray CT apparatus according to claim 1, wherein said transmitted X-ray image is the image which is obtained by installing a phantom in the vicinity of the rotation center of said scanner in order to detect said phantom; said phantom includes a columnar body which has the axial direction roughly in the same direction as that of the rotation center of said scanner, and a support member for holding said columnar body formed into a column-like shape; and the X-ray absorption coefficient of said columnar body is larger than that of said support member.
 3. An X-ray CT apparatus comprising: a scanner mounted with a detection system having an X-ray source for generating X-rays applied radially to an object and detection means arranged so as to be opposite to said X-ray source and adapted to detect the image of the transmitted X-rays transmitted through said object; rotation means for rotating said scanner around said object; reconstruction means for reconstructing a three-dimensional X-ray absorption coefficient distribution image of said object from said transmitted X-ray image; and estimate means for estimating the rotation-axis projection position on the basis of the X-ray absorption coefficient distribution image which is reconstructed by changing said rotation-axis projection position as the position of the rotation center of said scanner projected on a detection plane of a two-dimensional detector constituting said detection means.
 4. An X-ray CT apparatus according to claim 3, wherein said estimate means includes: initial-value decision means for deciding an initial value of said rotation-axis projection position; partial reconstruction means for on the basis of said rotation-axis projection position decided by said initial-value decision means, reconstructing a part of said X-ray absorption coefficient distribution image; and evaluation-function calculation means for evaluating the contrast of said X-ray absorption coefficient distribution image reconstructed by said partial reconstruction means to obtain a value of an evaluation function; and optimizing means for on the basis of the value of said evaluation function which is obtained by changing said rotation-axis projection position, estimating said rotation-axis projection position.
 5. An X-ray CT apparatus according to claim 4, wherein said initial-value decision means decides as said initial value the position at the center of an aperture width of a two-dimensional detector constituting said detection means, said aperture width corresponding to a rotation tangential direction of said scanner.
 6. An X-ray CT apparatus according to claim 4, wherein said partial reconstruction means reconstructs said X-ray absorption efficient distribution image on a rotation orbit plane of said X-ray source from said transmitted X-ray image.
 7. An X-ray CT apparatus according to claim 4, wherein said evaluation function is a maximum value of the CT value within a predetermined area of said X-ray absorption coefficient distribution image, and wherein said apparatus further comprises: rendering means for generating an X-ray tomographic image or/and a three-dimensional X-ray image of said object from said three-dimensional X-ray absorption coefficient distribution image which is reconstructed in said rotation-axis projection position where said evaluation function shows a maximum; and display means for displaying the X-ray tomographic image or/and the three-dimensional X-ray image of said object.
 8. An X-ray CT apparatus according to claim 4, wherein said evaluation function is a difference between a maximum value and a minimum value of the CT value within a predetermined area of said X-ray absorption coefficient distribution image, and wherein said apparatus further comprises: imaging means for generating an X-ray tomographic image or/and a three-dimensional X-ray image of said object from said three dimensional X-ray absorption coefficient distribution image which is reconstructed in said rotation-axis projection position where said evaluation function shows a maximum; and display means for displaying the X-ray tomographic image or/and the three-dimensional X-ray image of said object.
 9. An X-ray CT apparatus according to claim 4, further comprising evaluation-area decision means for specifying a pixel position where the CT value in said X-ray absorption coefficient distribution image reconstructed by said partial reconstruction means becomes a maximum or a local maximum, wherein said optimizing means estimates said rotation-axis projection position from the area containing said pixel position.
 10. An X-ray CT apparatus according to claim 4, further comprising: pre-processing means for carrying out the pre-processing for said transmitted X-ray image on the rotation orbit plan of said X-ray source; and temporary storage means for storing said transmitted X-ray image which has been subjected to the pre-processing by said pre-processing means for a period of time when said rotation-axis projection position is estimated.
 11. A phantom for use in an X-ray CT apparatus defined in any one of claims 1 and 3 to 10, wherein a phantom is provided which comprises a columnar body which has the axial direction roughly in the same direction as that of the rotation center of said scanner and a support member for holding said columnar body which is formed into a columnar shape, and wherein the X-ray absorption coefficient of said columnar body is larger than that of said support member.
 12. An X-ray detecting method comprising: the steps of utilizing a scanner mounted with a detection system having an X-ray source for generating X-rays applied radially to an object and detection means arranged so as to be opposite to said X-ray source, collecting images of transmitted X-rays transmitted through said object; deciding previously the rotation-axis projection position which is the position where the rotation center of said scanner is projected on the detection plane of a two-dimensional detector constituting said detection means; on the basis of said rotation-axis projection position, reconstructing the X-ray absorption coefficient distribution image of said object from said transmitted X-ray image; specifying said rotation-axis projection position, where the contrast of said X-ray absorption coefficient distribution image becomes a maximum or a local maximum, as said rotation-axis projection position on said transmitted X-ray image; on the basis of said specified rotation axis projection position, reconstructing the three-dimensional X-ray absorption coefficient distribution image of said object from said transmitted X-ray image; generating the X-ray tomographic image or/and the three-dimensional X-ray image from said three-dimensional X-ray absorption coefficient distribution image; and displaying the X-ray tomographic image or/and the three-dimensional X-ray image of said object.
 13. An X-ray detecting method comprising: the steps of utilizing a scanner mounted with a detection system having an X-ray source for generating X-rays applied radially to an object and detection means arranged so as to be opposite to said X-ray source, collecting images of transmitted X-rays transmitted through said object; deciding previously the rotation-axis projection position which is the position where the rotation center of said scanner is projected on the detection plane of a two-dimensional detector constituting said detection means; on the basis of said rotation-axis projection position, reconstructing the X-ray absorption coefficient distribution image of said object from said transmitted X-ray image; estimating said rotation-axis projection position from said X-ray absorption coefficient distribution image; on the basis of said estimated rotation-axis projection position, reconstructing the three-dimensional X-ray absorption coefficient distribution image of said object from said transmitted X-ray image; generating the X-ray tomographic image or/and the three-dimensional X-ray image from said three-dimensional X-ray absorption coefficient distribution image; and displaying the X-ray tomographic image or/and the three-dimensional X-ray image of said object. 